1. Field of the Invention
The present invention relates to an optical recording, and more particularly, to an apparatus and a method of preventing deformation of a recorded mark due to at least one of a spherical aberration due to a thickness deviation of a recording medium, and a coma aberration due to a relative radial tilting and tangential tilting between the recording medium and an objective lens, so that a proper mark can be recorded.
2. Description of the Related Art
Generally, an information recording and/or reproducing density increases as the size of an optical spot focused on a recording medium by an optical pickup decreases. As shown in the Equation 1 below, the size of the optical spot is reduced as the wavelength λ of a used light beam gets shorter and the numerical aperture NA of an objective lens gets larger.Spot Size αλ/NA   (1)
Therefore, to obtain a high recording density, a short wavelength light source, such as a blue semiconductor laser, and an objective lens having a large NA should be used in the optical pickup to reduce the size of the optical spot focused on the recording medium. In this technical field, a standardization of a high density recording medium, which uses a blue light source and an objective lens having an NA of about 0.85, increases a recording capacity to more than 20 GB, and whose thickness is reduced to about 0.1 mm, i.e., a next generation DVD, is proceeding.
However, a spherical aberration W40 is proportional to the 4th power of the NA of the objective lens and a thickness deviation of the recording medium. Thus, the recording medium has to have as uniform a thickness as possible to use an objective lens having an NA of about 0.85. But, it is very difficult to manufacture a recording medium having a uniform thickness of about 0.1 mm. Therefore, the spherical aberration proportional to the thickness deviation of the recording medium should be compensated for to properly record a mark on the recording medium. Here, the thickness of the recording medium refers to a distance between a surface upon which light is incident and a recording surface of the recording medium.
Additionally, a coma aberration is proportional to the cube of the NA of the objective lens and is caused by tilting of the recording medium (radial tilting and tangential tilting of the recording medium relative to the objective lens). Thus, if an objective lens having an NA of about 0.85 is used, the coma aberration should be compensated for to properly record a mark on the recording medium.